Wrap tortillas in foil and place on a part of the barbecue away from direct heat to warm. Combine Cajun spice and flour, add prawns and toss. Mix avocado, tomatoes, lime juice and coriander together, and season with salt and pepper. Heat oil in a frypan (or you can brush a flat grill plate with oil), add prawns and cook for 1-2 minutes on each side or until just cooked. Arrange the tortillas, prawns, salsa and lime wedges on serving plates with the extra coriander. Serve sour cream on the side.